An organic electroluminescent device is a spontaneously light-emitting device which features higher brightness and higher legibility than those of the liquid crystal devices and enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of the Eastman Kodak Co. have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles for emitting light, and have put an organic electroluminescent device using organic materials into a practical use. They have attempted to laminate a layer of a fluorescent material, i.e., a tris(8-hydroxyquinoline) aluminum (hereinafter abbreviated as Alq3) that is capable of transporting electrons and a layer of an aromatic amine compound capable of transporting holes, to inject the electric charges of the two into the layer of the fluorescent material to emit light, and have attained a brightness of as high as 1000 cd/m2 or more with a voltage of not more than 10 V.
So far, very many improvements have been made to put the organic electroluminescent device (organic EL device) to practical use. At present, the organic electroluminescent device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing their roles than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and to utilize a phosphorescent luminous material as the luminous material.
In the organic EL device, the electric charges injected from the two electrodes recombine in the luminous layer to emit light. Here, what is important is how efficiently the two electric charges, i.e., holes and electrons, be handed over to the luminous layer. Upon improving the hole injection property and improving the electron blocking power for blocking the electrons injected through the cathode, it is made possible to improve the probability of recombination of the holes and the electrons. Upon confining the excitons formed in the luminous layer, further, it is allowed to attain a high luminous efficiency. Therefore, the role played by the hole-transporting material is important, and it has been urged to provide a hole-transporting material that has a hole injection property, a hole mobility, a high electron blocking power and a high durability against the electrons.
As for the life of the device, heat resistances of the materials forming the device and amorphousness thereof also play important roles. If the materials having low heat resistances are used, the materials undergo thermal decomposition due to the heat produced when the device is driven and are deteriorated. If the materials that are used have low amorphousness, the thin films thereof are crystallized in short periods of time and the device is deteriorated. Therefore, the materials used for forming the organic EL device must have high heat resistances and good amorphousness.
As the hole-transporting materials used so far for the organic EL devices, there have been known an N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (hereinafter abbreviated as NPD) and various aromatic amine derivatives (see, for example, patent documents 1 and 2).
The NPD has a favorable hole-transporting capability but has a glass transition point (Tg) which serves as an index of heat resistance of as low as 96° C. Under high temperature conditions, therefore, the device properties are deteriorated due to the crystallization. Further, some aromatic amine derivatives have high hole mobilities (e.g., 10−3 cm2/Vs or higher), namely, have excellent mobilities. Such compounds, however, have insufficient electron blocking powers; i.e., the electrons partly pass through the luminous layer, and improvement in the luminous efficiency cannot be expected.
In order to further improve the efficiency as described above, it has been desired to provide a material for the organic EL devices having a higher electron blocking power and having a higher stability and higher heat resistance in the form of a thin film.
As the material for the organic EL use improving heat stability and hole-transporting property, there have been proposed compounds having an indolocarbazole ring structure (e.g., see patent documents 3 and 4).
The devices forming the hole injection layer and the hole-transporting layer using these compounds, exhibit improved heat resistances and luminous efficiencies, which, however, are not still sufficient. Besides, they require high driving voltages and their current efficiencies are not sufficient, either. Therefore, it has been desired to lower the driving voltage and to improve the luminous efficiency.
As the compounds having the indolocarbazole ring structure, further, there have been proposed the compounds A and B having substituted indolo[2,3-a]carbazole ring structures represented by the following formulas (e.g., see patent documents 5 to 9).

However, these compounds are used as host materials of the luminous layer, but properties as the hole-transporting materials have not been quite described.